In recent years, a vibration actuator that rotates a rotor utilizing ultrasonic vibrations has been proposed and put into practical use. The vibration actuator generates elliptic motion or traveling waves on the surface of the stator using a piezoelectric element, and brings the stator in pressure contact with the rotor, to thereby move the rotor through the intermediation of a frictional force therebetween.
For example, Patent Document 1 discloses an actuator in which a spherical rotor is preloaded by a plurality of elastic bodies through bearings to bring the rotor in pressure contact with the stator, and in this state, a drive voltage is applied to a plurality of piezoelectric element plates that are superimposed on one another to generate ultrasonic vibrations in the stator, thereby rotating the rotor. Herein, “preload” refers to a pressure by which the rotor is pressed toward the stator in a state where at least the piezoelectric element is not energized.    Patent Document 1: JP 2004-312810 A